Resin-lined steel pipe and method for production thereof

ABSTRACT

A plastic lined steel pipe characterized by having an adhesive layer on an inner surface of a steel pipe, having a plastic layer on its further inner side, and having an initial shearing adhesion strength between the steel pipe and the plastic layer of 2.0 MPa or more, said steel pipe being a steel pipe given substrate treatment on its inner surface in advance, said substrate treatment comprising forming a phosphate chemical treatment coating treated for grain refinement, and preferably having an epoxy primer layer between the steel pipe and the adhesive layer. A method for producing a plastic lined steel pipe characterized by inserting a plastic pipe having an outside diameter smaller than the inside diameter of the steel pipe subjected to the substrate treatment and having an adhesive layer on its outer surface into the steel pipe, reducing the outside diameter of the steel pipe so as to make the plastic pipe closing contact it, and heating the result at a temperature not less than a melt end temperature of the adhesive layer and less than a melt start temperature of the plastic pipe.

TECHNICAL FIELD

The present invention relates to a plastic lined steel pipe used forpiping etc. for water supply, hot water supply, air-conditioning,firefighting, drainage, etc. and a method for producing the same, inmore detail relates to a plastic lined steel pipe excellent in adhesionbetween the steel pipe and the inner surface plastic lining layer over along period even at cold locations and a method for producing the same.

BACKGROUND ART

As the material of piping for transporting water etc., other than steelpipes such as forged steel pipes and seam-welded steel pipes, polyvinylchloride, polyethylene, polypropylene, polybutene, and otherthermoplastic plastic pipes are being used. Steel pipes have largermechanical strengths in comparison with these plastic pipes, thereforehave higher shock resistance at the time of installation and haveexcellent compression resistance even when buried under heavy trafficroads. Even when the temperature of the transported fluid is high, thepipes are sufficiently large and excellent in compression strength whencompared with plastic pipes and hard to burn unlike plastic pipes, sowill not burn by fire even when used for indoor purposes.

However, in applications requiring prevention of clouding of the fluidand the prevention of clogging of the pipeline due to the corrosion ofthe steel, use is made of plastic pipe not subject to corrosion. As apiping material having the merits of both, a composite pipe of plasticand steel prevented from corrosion by inserting a plastic pipe into theinner surface of a steel pipe is known. For example, as a water pipe anddrainage pipe, a composite pipe of steel and a soft polyvinyl chloridemaking good use of cheap polyvinyl chloride is being widely used, whileas a hot water pipe, a composite pipe of steel and a hard polyvinylchloride is being widely used.

When using a polyvinyl chloride material, however, there is also theproblem that dioxins are produced when incinerating the remaining piecesof composite pipes produced in on-site piping work. Accordingly, as thecomposite pipes used for water pipes, hot water pipes, drainage pipes,etc., pipes not using polyvinyl chloride have been desired.

Therefore, Japanese Unexamined Patent Publication (Kokai) No. 2001-9912and Japanese Unexamined Patent Publication (Kokai) No. 2001-9913disclose the method of lining the inside surface of a steel pipe byutilizing the shape memory of a polyolefin resin or cross-linkedpolyolefin resin free from the problem of production of dioxins torestore by heat a polyolefin plastic pipe or cross-linked polyolefinplastic pipe reduced in diameter from the inside diameter of the steelpipe.

When lining the inside surface of a steel pipe by a polyolefin plasticpipe or a cross-linked polyolefin plastic pipe by the method disclosedin Japanese Unexamined Patent Publication (Kokai) No. 2001-9912 andJapanese Unexamined Patent Publication (Kokai) No. 2001-9913, however,since a polyolefin resin or a cross-linked polyolefin resin is muchlarger in thermal shrinkage than steel in comparison with a polyvinylchloride, in the final cooling step of the production, the outsidediameter of the polyolefin plastic pipe or the cross-linked polyolefinplastic pipe tends to become smaller than the inside diameter of thesteel pipe, so a large peeling force acts at the interface between thesteel pipe and the polyolefin plastic pipe or the cross-linkedpolyolefin plastic pipe. For this reason, in a composite pipe of steeland a polyolefin resin or a cross-linked polyolefin resin, a peelingstress constantly acts upon the interface between the steel pipe and theplastic pipe. Therefore, even in a case where an adhesive, an epoxyprimer, or a chemical treatment coating is used for preventing peeling,if this is used for piping for water supply, hot water supply,air-conditioning, firefighting, drainage, etc. over a long period, thebonded interface between the steel pipe and the polyolefin plastic pipeor the cross-linked polyolefin plastic pipe deteriorates and theadhesion strength becomes weak, so there was the problem that thepolyolefin plastic pipe or the cross-linked polyolefin plastic pipewould peel off from the inner surface of the steel pipe due to theshrinkage stress inherent in the polyolefin plastic pipe or thecross-linked polyolefin plastic pipe. Further, when considering use atcold locations, the polyolefin resin or the cross-linked polyolefinresin further tends to shrink and the peeling force to become larger. Itwas learned that a chemical treatment coating was not durable againstthat and ended up breaking.

DISCLOSURE OF THE INVENTION

In consideration with the above problems, the present invention providesa plastic lined steel pipe used for piping for water supply, hot watersupply, air-conditioning, firefighting, drainage, etc. which isexcellent in adhesion between the steel pipe and the inner surfaceplastic lining layer over a long period even at cold locations and amethod for producing the same.

The inventors took note of polyolefin resins and cross-linked polyolefinresins which are free from the problem of production of dioxins.Further, since these resins have a thermal shrinkage far larger thanthat of steel in comparison with polyvinyl chloride, they proposed notto utilize the shape memory property. Namely, in the method of liningthe inside surface by restoring by heating a plastic pipe reduced indiameter to be smaller than the inside diameter of the steel pipe, theoutside diameter of the plastic pipe tends become smaller than theinside diameter of the steel pipe in the final cooling step of theproduction, so a large peeling force acts upon the interface of thesteel pipe and the plastic pipe. The present invention was made by thediscovery that by conversely drawing the steel pipe so as to line theinside surface while leaving an expansion force whereby the plastic pipetries become larger in outside diameter than the inside diameter of thesteel pipe and further providing between the steel pipe and the plasticpipe an adhesive layer and a phosphate chemical treatment coatingtreated for grain refinement to reinforce the adhesion and, according toneed, providing an epoxy primer layer, a plastic lined steel pipeexcellent in adhesion between the steel pipe and the inner surfaceplastic lining layer over a long period even at cold locations andusable for piping for water supply, hot water supply, air-conditioning,firefighting, drainage, etc. was possible. The gist thereof is asfollows:

(1) A plastic lined steel pipe characterized by having an adhesive layeron an inner surface of a steel pipe or a steel pipe galvanized on itsouter surface, having a plastic layer on its further inner side, andhaving an initial shearing adhesion strength between the steel pipe andthe plastic layer of 2.0 MPa or more, said steel pipe being a steel pipegiven substrate treatment on its inner surface in advance, saidsubstrate treatment comprising forming a phosphate chemical treatmentcoating treated for grain refinement.

Here, the “initial shearing adhesion strength” means the adhesionstrength between the steel pipe and the plastic layer after adhesion andbefore use. When this initial shearing adhesion strength is less than2.0 MPa, the resin lining is liable to peel off during use, so theinitial shearing adhesion strength must be 2.0 MPa or more, preferably4.0 MPa or more.

(2) A plastic lined steel pipe as set forth in the above (1), whereinsaid plastic layer is a polyolefin resin or a cross-linked polyolefinresin.

(3) A plastic lined steel pipe as set forth in the above (1) or (2)wherein said adhesive layer is comprised of one or more of a maleicanhydride-modified polyolefin, itaconic anhydride-modified polyolefin,ethylene/maleic anhydride copolymer, ethylene/maleic anhydride/acrylatecopolymer, ethylene/maleic anhydride/acrylate ester copolymer,ethylene/acrylate copolymer, ethylene/acrylate ester copolymer,ethylene/methacrylate copolymer, ethylene/vinyl acetate copolymer, andionomer, and a melt end temperature of the adhesive layer is over ausage temperature of said plastic layer and less than a melt starttemperature.

(4) A plastic lined steel pipe as set forth in any one of the above (1)to (3), wherein an epoxy primer layer is provided between said steelpipe and said adhesive layer.

(5) A plastic lined steel pipe as set forth in any one of the above (1)to (4), wherein a primary anti-rust coating, a zinc rich paint coating,or a polyolefin coating is provided on the outer surface of said plasticlined steel pipe instead of galvanization.

(6) A method for producing a plastic lined steel pipe as set forth inany one of the above (1) to (5), comprising, when producing said plasticlined steel pipe, applying substrate treatment to a steel pipe orapplying substrate treatment to a steel pipe, then applying an epoxyprimer layer, inserting a plastic pipe having an outside diametersmaller than the inside diameter of the steel pipe and having anadhesive layer on its outer surface into said steel pipe, drawing thesteel pipe so as to make the plastic pipe come in close contact with thesteel pipe, then heating the result at a temperature not less than amelt end temperature of the adhesive layer and less than a melt starttemperature of the plastic pipe.

(7) A method for producing a plastic lined steel pipe as set forth inthe above (6) further comprising, when drawing said steel pipe, drawingthe steel pipe so that the outside diameter of the plastic pipe isreduced by 0.5 to 10%.

BEST MODE FOR WORKING THE INVENTION

When producing the plastic lined steel pipe of the present invention,first the inner surface of the steel pipe is degreased and pickled orblasted to clean it. This steel pipe may be treated on its outer surfacewith hot dip galvanization or other plating. One with an outsidediameter of about 10 to 2000 mm, usually about 20 to 170 mm, is used.

Next, the inventors discovered that if applying, as the substratetreatment of the steel pipe, a phosphate chemical treatment coatingtreated for grain refinement to reinforce its adhesion strength, even ifthe plastic pipe tries to shrink further at a cold location and thepeeling force becomes larger, the chemical treatment coating will neverfail to withstand this and end up breaking. Further, they discoveredthat the finder the grains of the phosphate of the chemical treatmentcoating, the more improved the adhesion strength. As the chemicaltreatment solution, a mixture comprised of for example phosphoric acid,nitric acid, zinc oxide, and calcium carbonate and water and adjusted inpH by sodium hydroxide (calcium zinc phosphate treatment solution) isused. Calcium zinc phosphate is excellent in heat resistance, so ispreferred for the present invention which is accompanied with heating inthe production. A good waterproof adhesion is obtained, when the amountsof addition of these are 8 to 15 g/L as phosphoric acid ions, 30 to 60g/L as nitric acid ions, 2 to 4 g/L as zinc ions, 5 to 10 g/L as calciumions, and the pH is in the range of 2.0 to 2.5. As a representativecalcium zinc phosphate treatment solution corresponding to the abovecomposition, there is Palbond P (made by Nihon Parkerizing Co. Ltd.)

In the coating of the chemical treatment coating, the steel pipe may becoated with the above chemical treatment solution by dipping, injectioninto the steel pipe, or spraying, then heated and dried by hot airheating, high frequency induction heating, etc. The amount of depositionof this chemical treatment coating is preferably about 1 to 10 g/m². Ifthe deposition amount thereof is less than 1 g/m², the chemicaltreatment coating will not completely cover the iron surface, while ifit is over 10 g/m², brittle secondary crystal grains will grow in thechemical treatment coating, so the waterproof adhesion strength of theplastic lining layer will be lowered.

The grain refinement is carried out, before coating the chemicaltreatment coating, by dip coating, insertion coating, or spray coatingthe steel pipe with for example a treatment solution obtained bydispersing titanium colloid in water in a range of from 1 to 5 g/L (asrepresentative example, there is Prepalene Z (made by Nihon ParkerizingCo. Ltd.)) and/or adding to the above chemical treatment solution forexample basic nickel carbonate as the nickel ions in a range of from 0.2to 1.0 g/L. The titanium or nickel forms cores for the precipitation ofcrystal grains of the phosphate and densely adhere to the iron surfaceto refine the grains, therefore the contact area between the crystalgrains and the iron increases and the adhesion strength is improved. Ifnot performing the grain refinement, crystal grains having a size ofover 10 μm will be generated, but if performing the grain refinement,the crystal grains will be refined to a size of 10 μm or less, thereforethe adhesion strength is improved three-fold or more. If the amountsadded are less than the lower limits, the effect of the grain refinementwill be lowered, while if over the upper limits, the economicalness willdeteriorate.

After this, a plastic pipe having an outside diameter smaller than theinside diameter of the steel pipe and longer than the length of thesteel pipe is inserted into the steel pipe, the steel pipe is rolldrawn, strike drawn, or die drawn so that the outside diameter of theplastic pipe is reduced by 0.5 to 10% to thereby make the plastic pipeclosely contact the inner surface of the steel pipe. If the reductionratio of this plastic pipe is less than 0.5%, the expansion force of theplastic pipe becomes small, so the adhesion strength of the plasticlining layer will be lowered. If the reduction ratio of the plastic pipeis over 10%, the plastic pipe will deform, so the adhesion with theinner surface of the steel pipe will be degraded.

As the plastic pipe used in the present invention, use is made of aplastic pipe made of a polyolefin resin or a cross-linked polyolefinresin. As the polyolefin resin, use is made of an ethylene homopolymeror an ethylene/α-olefin copolymer obtained by copolymerizing ethyleneand propylene, 1-butene, 1-hexene, 1-octene, or another α-olefin or amixture of the same into which additives such as an antioxidant, UVabsorbent, fire retardant, pigment, filler, lubricant, antistatic agentand other resins are mixed according to need within a range notimpairing the performance of the present invention.

As the cross-linked polyolefin resin, use is made of a polyolefin resinwhich is cross-linked by using a radical generator or a silane-modifiedpolyolefin resin which is water cross-linked (silane cross-linked). Asthe radical generator, use is made of an organic peroxide such asdicumyl peroxide, benzoyl peroxide, di-t-butyl peroxide, or2,5-dimethyl-2,5-di(t-butylperoxy)hexane. Further, other than the aboveorganic peroxides, use can be also made of an azo compound such asazoisobutylonitrile. The silane modification is carried out by graftreacting an ethylenic unsaturated silane compound with the polyolefinresin in the presence of the radical generator. Here, the ethylenicunsaturated silane compound is represented by the following generalformula:RSiR′_(n)Y_(3-n)wherein, R represents an ethylenic unsaturated hydrocarbon group orhydrocarbon oxy group, R′ represents an aliphatic saturated hydrocarbongroup, Y represents an organic group which can be hydrolyzed, and nrepresents 0 to 2.

Specifically, vinyl trimethoxysilane, vinyl triethoxysilane, vinyltriacetoxysilane, etc. is used. This silane modification may be carriedout in advance by an extruder etc. or may be carried out by at the timeof shaping by charging the stock ingredients from a hopper andperforming it at the kneading portion of the shaping machine. Thecross-linking reaction is carried out by heat treatment, watertreatment, etc. at the time of the extrusion and/or shaping. In the caseof a silane-modified polyolefin resin, in order to improve thecross-linking speed, a silanol condensation catalyst is preferably usedtogether. This may be mixed in at the time of the shaping or coatedafter the shaping. As the silanol condensation catalyst, dibutyl tindilaurate, dioctyl tin dilaurate, cobalt naphthenate, toluene sulfonicacid, etc. can be used. The cross-linked polyolefin resin used in thepresent invention may have added to it, within a range not impairing theperformance of the present invention, an additive such as anantioxidant, UV absorbent, fire retardant, pigment, filler, lubricant,or antistatic agent or other resin according to need.

As the method of preparation of the plastic pipe used in the presentinvention, a resin is extruded in the form of a pipe using an extruderor the like from a round die having an outside diameter smaller than theinside diameter of the steel pipe to be lined, then cooled to fix itsshape. The thickness of this plastic pipe can be freely set according toneed. It is not particularly limited, but usually a pipe of a thicknessof 0.3 mm to 10 mm, preferably 0.5 mm to 5 mm, is used. Further, inorder to improve the adhesion strength with the adhesive layer, aftershaping the plastic pipe, according to need, the outer surface may becoated by a commercially available primer, oxidized, or roughened.

A steel pipe and a plastic pipe do not have much adhesiveness, so anadhesive layer is desirably provided between them. Especially, it wasfound that by forming the adhesive layer by a material comprised of oneor two or more of a maleic anhydride-modified polyolefin, itaconicanhydride-modified polyolefin, ethylene/maleic anhydride copolymer,ethylene/maleic anhydride/acrylate copolymer, ethylene/maleicanhydride/acrylate ester copolymer, ethylene/acrylate copolymer,ethylene/acrylate ester copolymer, ethylene/methacrylate copolymer,ethylene/vinyl acetate copolymer, and ionomer and having a melt endtemperature less than the melt start temperature of the plastic pipe andover the usage temperature of the plastic pipe, an adhesion strength farsuperior to that of other materials is manifested. As the polyolefin ofan adhesive layer made of a maleic anhydride-modified polyolefin, use ismade of for example a low crystallinity ethylene-based polymer having amelt end temperature of 100° C. If the melt end temperature is not lessthan the melt start temperature of the plastic pipe, it is necessary toperform heating at a temperature not less than the melt starttemperature of the plastic pipe for manifesting the adhesion strength,therefore the plastic pipe will soften, the expansion force will belost, the pipe will deform. Further, if the melt end temperature is notmore than the usage temperature, the adhesive layer will completely meltduring use, so the adhesion strength of the plastic lining layer will belowered.

The adhesive layer is coated by coextruding the adhesive layer onto theouter surface of the plastic pipe at the time of shaping the plasticpipe using a two-layer round die having an outside diameter smaller thanthe inside diameter of the steel pipe to be lined or by coextruding theadhesive layer after shaping the plastic pipe by using a round die orT-die. Further, in order to manifest the adhesion strength, after rolldrawing, forge drawing, or die drawing the steel pipe, the pipe isheated at a temperature not less than the melt end temperature of theadhesive layer and less than the melt start temperature of the plasticpipe by hot air heating, high frequency induction heating, etc. If theheating temperature is less than the melt end temperature of theadhesive layer, the adhesive layer will not be completely melted, so theadhesion strength will not be manifested. Further, if the heatingtemperature is the melt start temperature of the plastic pipe or more,the plastic pipe will soften, the expansion force will be lost, and thepipe will deform. The thickness of this adhesive layer can be freely setaccording to need. It is not particularly limited, but usually athickness of 1 μm to 3 mm, preferably 10 μm to 1.5 mm, is used.

If there is an epoxy primer layer between the steel pipe and theadhesive layer, a good waterproof adhesiveness is obtained, so this isdesirable. As the epoxy primer layer, a mixture formed by for example anepoxy, a pigment, an additive, and a curing agent (epoxy resin powderprimer) is used. As the epoxy, for example, a diglycidyl ether ofbisphenol A, a diglycidyl ether of bisphenol F, or a phenol novolac typeor cresol novolac type glycidyl ether is used. These epoxys can be usedalone or can be used mixed together according to the object. As thepigment, a fine powder of silica, barium sulfate, calcium carbonate, orother extender pigment or titanium oxide, carbon black, or othercoloring pigment is used. A good waterproof adhesiveness is obtainedwhen the amount added of these pigments is within a range of from 3 to50 parts by weight with respect to 100 parts by weight of the epoxy. Asthe additive, use can be made of an acryl oligomer, fine powder silicaor the like.

As the curing agent, a dibasic acid such as dicyandiamide or decanedicarbonate, a hydrazine such as adipic acid dihydrazide, an acidanhydride such as tetrahydrophthalate anhydride, a phenol-based curingagent obtained by adding bisphenol A to a diglycidyl ether of bisphenolA, or an amine adduct obtained by adding diamide diphenylmethane to adiglycidyl ether of the bisphenol A can be used. If using a dibasicacid, hydrazine, or phenol-based curing agent for the curing agent, theamount of the curing agent is determined by the ratio between theequivalent weight of the epoxy and the equivalent weight of the activehydrogen of the curing agent. As the equivalent weight ratio, an 0.6 to1.2 equivalent weight of the active hydrogen with respect to an 1.0equivalent weight of epoxy is good.

If using dicyandiamide as the curing agent, in order to lower the curingtemperature, a modified imidazole is added as the curing accelerator. Asthis modified imidazole, for example 2-methylimidazole,2-phenylimidazole, etc. can be utilized. For the blending of the curingagent in this case, a good waterproof adhesiveness is obtained ifdicyandiamide is added in a range of from 3 to 10 parts by weight withrespect to 100 parts by weight of the epoxy and the modified imidazoleis added in a range of from 0.1 to 3 parts by weight with respect to 100parts by weight of the epoxy. Similarly, even if using a phenol-basedcuring agent, a modified imidazole is effectively used as the curingaccelerator. As a representative epoxy resin powder paint correspondingto the above composition, there is Powdax E (made by Nippon Paint Co.Ltd.)

The epoxy primer layer may be coated by electrostatic spray coating orfluid suction coating the epoxy primer layer on the inner surface of thesteel pipe at room temperature to about 80° C, then heating the steelpipe to cure the layer at about 140 to 220° C. by hot air heating orhigh frequency induction heating. The thickness of this epoxy primerlayer is preferably 40 to 600 μm. If the thickness is less than 40 μm,there is possibility that the thickness become the film forming limit ofthe powder coating or less, so continuous coating will not be carriedout and therefore the waterproof adhesion strength of the plastic lininglayer will be lowered. Further, from the viewpoints of work efficiencyand economy, the upper limit of the thickness is preferably about 600μm.

It is also possible to provide the outer surface of the inner surfaceplastic lined steel pipe with a primary anti-rust coating, zinc richpaint coating, or polyolefin coating in place of the galvanization. Asthe primary anti-rust coating, a general commercially availablealkyd-based or epoxy-based paint etc. is coated to a thickness of about20 to 30 μm. As the zinc rich paint coating, a general commerciallyavailable organic or inorganic zinc rich paint etc. is coated to athickness of about 65 to 85 μm. Further, in order to improve thecorrosion resistance, it is also possible to coat a commerciallyavailable clear paint, white rust prevention paint, or the likeaccording to need after coating the zinc rich paint. If using apolyolefin coating, first the outer surface of the steel pipe isdegreased and blasted or pickled to clean it. Thereafter, the adhesiveand the polyolefin resin are sequentially coated.

As the adhesive, a material comprised of one or two or more of a maleicanhydride-modified polyolefin, itaconic anhydride-modified polyolefin,ethylene/maleic anhydride copolymer, ethylene/maleic anhydride/acrylatecopolymer, ethylene/maleic anhydride/acrylate ester copolymer,ethylene/acrylate copolymer, ethylene/acrylate ester copolymer,ethylene/methacrylate copolymer, ethylene/vinyl acetate copolymer, andionomer is used. As the rate of addition of the maleic anhydride, a goodadhesion strength is obtained when it is added within the range of from0.05 to 0.5 wt %.

The adhesive is coated by extrusion onto the outer surface of the steelpipe by using a round die or T-die. When the thickness of this adhesiveis about 80 to 400 μm, a good adhesion strength is obtained.

As the polyolefin resin, an ethylene homopolymer or an ethylene/α-olefincopolymer obtained by copolymerizing ethylene and propylene, 1-butene,1-hexene, 1-octene, or another α-olefin or a mixture of the sameincluding, according to need, an additive such as an antioxidant, UVabsorbent, fire retardant, pigment, filler, lubricant, or antistaticagent and another resin is used.

These polyolefin resins are coated by extrusion onto the outer surfaceof a steel pipe coated with an adhesive by using a round die or T-die,but the method of using a two-layer round die or two-layer T die andcoextruding the adhesive and the polyolefin resin for coating can alsobe used. When the thickness of this polyolefin resin is about 0.3 to 10μm, a good anti-corrosion property is obtained.

Further, when there is a chemical treatment coating or epoxy primerbetween the steel pipe and the adhesive, a good waterproof adhesivenessis obtained, so this is desirable. As the chemical treatment solution, amixture comprised of for example phosphoric acid, nitric acid, zincoxide, calcium carbonate and water and adjusted in pH by sodiumhydroxide (calcium zinc phosphate treatment solution) is used. In thecoating of the chemical treatment coating, the steel pipe may be coatedwith the above chemical treatment solution by spraying or dipping, thenheated and dried by high frequency induction heating, hot air heating,etc. The amount of deposition of this chemical treatment coating ispreferably about 1 to 10 g/m². If the deposition amount thereof is lessthan 1 g/m² or over 10 g/m², the waterproof adhesion strength of thepolyolefin coating will be lowered. Further, grain refinement may alsobe performed.

As the epoxy primer, for example an epoxy resin powder primer is used.The epoxy primer layer may be coated by pre-heating the steel pipe giventhe chemical treatment coating by high frequency induction heating orhot air heating, then electrostatic spray coating or fluid suctioncoating the epoxy primer layer on the surface. The thickness of thisepoxy primer layer is preferably 40 to 600 μm. If the thickness is lessthan 40 μm, the waterproof adhesion strength of the polyolefin coatingis lowered. Further, from the viewpoints of the work efficiency andeconomy, the upper limit of the thickness is preferably about 600 μm.

The present invention will be explained in detail next based onexamples.

EXAMPLE 1

A steel pipe having an outside diameter of 50.8 mm, a thickness of 3.3mm, and a length of 3930 mm was degreased by a commercially availablealkali degreasing agent and pickled to remove the rust, then the steelpipe was sequentially dipped in a treatment solution obtained bydispersing titanium colloid in water (Prepalene Z made by NihonParkerizing Co. Ltd.) and a calcium zinc phosphate treatment solution(Palbond P made by Nihon Parkerizing Co. Ltd.) and dried by hot airheating to form a chemical treatment coating. The amount of depositionof the chemical treatment coating was 4 g/m², and the average grain sizethereof was about 5 μm. Next, using a two-layer round die, when shapinga polyethylene plastic pipe (melt start temperature of 120° C.) havingan outside diameter of 42.2 mm, a thickness of 1.5 mm, and a length of4040 mm, an adhesive made of a maleic anhydride-modified polyethylene(melt end temperature: 100° C.) was coated on the outer surface bycoextrusion so as to form an adhesive layer. The thickness of theadhesive layer was 200 μm.

Thereafter, the polyethylene plastic pipe was inserted into the steelpipe and the steel pipe was roll drawn so that the outside diameter ofthe polyethylene plastic pipe was reduced by 1.4%, whereby thepolyethylene plastic pipe was made to closely contact the inner surfaceof the steel pipe, then the result was heated to 115° C. in a hot airheating furnace. The part of the polyethylene plastic pipe protrudingfrom the end portion of the steel pipe was cut off. The outer surface ofthis inner surface plastic lined steel pipe was degreased by acommercially available alkali degreasing agent, grit blasted to removethe rust, then coated with a commercially available organic zinc richpaint to a thickness of 75 μm and further coated with a commerciallyavailable clear paint to a thickness of 30 μm.

EXAMPLE 2

The inner surface of the steel pipe hot dip galvanized on its outersurface and having an outside diameter of 50.8 mm, a thickness of 3.3mm, and a length of 3930 mm was degreased by a commercially availablealkali degreasing agent and pickled to remove the rust, thensuccessively injected with a treatment solution obtained by dispersingtitanium colloid in water (Prepalene Z made by Nihon Parkerizing Co.Ltd.) and a calcium zinc phosphate treatment solution (Palbond P made byNihon Parkerizing Co. Ltd.), and dried by hot air heating to form achemical treatment coating. The amount of deposition of the chemicaltreatment coating was 4 g/m². Next, an epoxy resin powder primer (PowdaxE made by Nippon Paint Co. Ltd.) was coated on the inner surface of thesteel pipe at room temperature by electrostatic spraying, and the resultwas heated to 180° C. in a hot air heating furnace to form an epoxyprimer layer. The thickness of the epoxy primer layer was 100 μm.Further, using a two-layer round die, when shaping a polyethyleneplastic pipe having an outside diameter of 42.4 mm, a thickness of 1.5mm, and a length of 4040 mm (melt start temperature of 120° C.), anadhesive made of a maleic anhydride-modified polyethylene (melt endtemperature: 100° C.) was coated on the outer surface by coextrusion toform an adhesive layer. The thickness of the adhesive layer was 200 μm.

Thereafter, the polyethylene plastic pipe was inserted into the steelpipe and the steel pipe was roll drawn so that the outside diameter ofthe polyethylene plastic pipe was reduced by 1.4%, whereby thepolyethylene plastic pipe was made to closely contact the inner surfaceof the steel pipe, then the result was heated to 115° C. in a hot airheating furnace. The part of the polyethylene plastic pipe protrudingfrom the end portion of the steel pipe was cut off.

EXAMPLE 3

A steel pipe having an outside diameter of 50.8 mm, a thickness of 3.3mm, and a length of 3930 mm was degreased by a commercially availablealkali degreasing agent and pickled to remove the rust, then the steelpipe was sequentially dipped in a treatment solution obtained bydispersing titanium colloid in water (Prepalene Z made by NihonParkerizing Co. Ltd.) and a calcium zinc phosphate treatment solution(Palbond P made by Nihon Parkerizing Co. Ltd.) and dried by hot airheating to form a chemical treatment coating. The amount of depositionof the chemical treatment coating was 4 g/m². Next, an epoxy resinpowder primer (Powdax E made by Nippon Paint Co. Ltd.) was coated on theinner surface of the steel pipe at room temperature by electrostaticspraying, then the result was heated to 180° C. in a hot air heatingfurnace to form an epoxy primer layer. The thickness of the epoxy primerlayer was 100 μm. Further, using a two-layer round die, when shaping apolyethylene plastic pipe having an outside diameter of 42.4 mm, athickness of 1.5 mm, and a length of 4040 mm (melt start temperature of120° C.), an adhesive made of a maleic anhydride-modified polyethylene(melt end temperature: 100° C.) was coated on the outer surface bycoextrusion to form an adhesive layer. The thickness of the adhesivelayer was 200 μm.

Thereafter, the polyethylene plastic pipe was inserted into the steelpipe and the steel pipe was roll drawn so that the outside diameter ofthe polyethylene plastic pipe was reduced by 1.4%, whereby thepolyethylene plastic pipe was made to closely contact the inner surfaceof the steel pipe, then the result was heated to 115° C. in a hot airheating furnace. The part of the polyethylene plastic pipe protrudingfrom the end portion of the steel pipe was cut off. The outer surface ofthis inner surface plastic lined steel pipe was degreased by acommercially available alkali degreasing agent, grit blasted to removethe rust, then coated with a commercially available alkyd-based paint toa thickness of 25 μm.

EXAMPLE 4

A steel pipe having an outside diameter of 50.8 mm, a thickness of 3.3mm, and a length of 3930 mm was degreased by a commercially availablealkali degreasing agent and pickled to remove the rust, then the steelpipe was sequentially dipped in a treatment solution obtained bydispersing titanium colloid in water (Prepalene Z made by NihonParkerizing Co. Ltd.) and a calcium zinc phosphate treatment solution(Palbond P made by Nihon Parkerizing Co. Ltd.) and dried by hot airheating to form a chemical treatment coating. The amount of depositionof the chemical treatment coating was 4 g/m². Next, an epoxy resinpowder primer (Powdax E made by Nippon Paint Co. Ltd.) was coated on theinner surface of the steel pipe at room temperature by electrostaticspraying, and the result was heated to 180° C. in hot air heatingfurnace to form an epoxy primer layer. The thickness of the epoxy primerlayer was 100 μm. Further, using a two-layer round die, when shaping apolyethylene plastic pipe having an outside diameter of 42.4 mm, athickness of 1.5 mm, and a length of 4040 mm. (melt start temperature of120° C.), an adhesive made of a maleic anhydride-modified polyethylene(melt end temperature: 100° C.) was coated on the outer surface bycoextrusion to form an adhesive layer. The thickness of the adhesivelayer was 200 μm.

Thereafter, the polyethylene plastic pipe was inserted into the steelpipe and the steel pipe was roll drawn so that the outside diameter ofthe polyethylene plastic pipe was reduced by 1.4%, whereby thepolyethylene plastic pipe was made to closely contact the inner surfaceof the steel pipe, then the result was heated to 115° C. in a hot airheating furnace. The part of the polyethylene plastic pipe protrudingfrom the end portion of the steel pipe was cut off. The outer surface ofthis inner surface plastic lined steel pipe was degreased by acommercially available alkali degreasing agent, grit blasted to removethe rust, then coated with a commercially available organic zinc richpaint to a thickness of 75 μm and further coated with a commerciallyavailable clear paint to a thickness of 30 μm.

EXAMPLE 5

A steel pipe having an outside diameter of 50.8 mm, a thickness of 3.3mm, and a length of 3930 mm was degreased by a commercially availablealkali degreasing agent and pickled to remove the rust, then the steelpipe was sequentially dipped in a treatment solution obtained bydispersing titanium colloid in water (Prepalene Z made by NihonParkerizing Co. Ltd.) and a calcium zinc phosphate treatment solution(Palbond P made by Nihon Parkerizing Co. Ltd.) and dried by hot airheating to form a chemical treatment coating. The amount of depositionof the chemical treatment coating was 4 g/m². Next, an epoxy resinpowder primer (Powdax E made by Nippon Paint Co. Ltd.) was coated on theinner surface of the steel pipe at room temperature by electrostaticspraying, then the result was heated to 180° C. in a hot air heatingfurnace to form an epoxy primer layer. The thickness of the epoxy primerlayer was 100 μm. Further, using a two-layer round die, when shaping apolyethylene plastic pipe having an outside diameter of 42.4 mm, athickness of 1.5 m, and a length of 4040 mm (melt start temperature of120° C.), an adhesive made of a maleic anhydride-modified polyethylene(melt end temperature: 100° C.) was coated on the outer surface bycoextrusion to form an adhesive layer. The thickness of the adhesivelayer was 200 μm.

Thereafter, the polyethylene plastic pipe was inserted into the steelpipe and the steel pipe was roll drawn so that the outside diameter ofthe polyethylene plastic pipe was reduced by 1.4%, whereby thepolyethylene plastic pipe was made to closely contact the inner surfaceof the steel pipe, then the result was heated to 115° C. in a hot airheating furnace. The part of the polyethylene plastic pipe protrudingfrom the end portion of the steel pipe was cut off.

The outer surface of the inner surface plastic lined steel pipe wasdegreased by a commercially available alkali degreasing agent, gritblasted to remove the rust, then coated with a calcium zinc phosphatetreatment solution by spraying and heated to a steel pipe surfacetemperature of 115° C. by high frequency induction heating to form achemical treatment coating. The amount of deposition of the chemicaltreatment coating was 4 g/m². Immediately after that, using a two-layerround die, a maleic anhydride-modified polyethylene adhesive and apolyethylene resin were coated by coextrusion. The thicknesses of themaleic anhydride-modified polyethylene adhesive and the polyethyleneresin were 200 μm and 1.0 mm.

EXAMPLE 6

A steel pipe having an outside diameter of 50.8 mm, a thickness of 3.3mm, and a length of 3930 mm was degreased by a commercially availablealkali degreasing agent and pickled to remove the rust, then the steelpipe was sequentially dipped in a treatment solution obtained bydispersing titanium colloid in water (Prepalene Z made by NihonParkerizing Co. Ltd.) and a calcium zinc phosphate treatment solution(Palbond P made by Nihon Parkerizing Co. Ltd.) and dried by hot airheating to form a chemical treatment coating. The amount of depositionof the chemical treatment coating was 4 g/m². Next, an epoxy resinpowder primer (Powdax E made by Nippon Paint Co. Ltd.) was coated on theinner surface of the steel pipe at room temperature by electrostaticspraying, then the result was heated to 180° C. in a hot air heatingfurnace to form an epoxy primer layer. The thickness of the epoxy primerlayer was 100 μm. Further, using a two-layer round die, when shaping apolyethylene plastic pipe having an outside diameter of 42.4 mm, athickness of 1.5 mm, and a length of 4040 mm (melt start temperature of120° C.), an adhesive made of an itaconic anhydride-modifiedpolyethylene (melt end temperature: 100° C.) was coated on the outersurface by coextrusion to form an adhesive layer. The thickness of theadhesive layer was 200 μm.

Thereafter, the polyethylene plastic pipe was inserted into the steelpipe and the steel pipe was roll drawn so that the outside diameter ofthe polyethylene plastic pipe was reduced by 1.4%, whereby thepolyethylene plastic pipe was made to closely contact the inner surfaceof the steel pipe, then the result was heated to 115° C. in a hot airheating furnace. The part of the polyethylene plastic pipe protrudingfrom the end portion of the steel pipe was cut off. The outer surface ofthis inner surface plastic lined steel pipe was degreased by acommercially available alkali degreasing agent, grit blasted to removethe rust, then coated with a commercially available organic zinc richpaint to a thickness of 75 μm and further coated with a commerciallyavailable clear paint to a thickness of 30 μm.

EXAMPLE 7

A steel pipe having an outside diameter of 50.8 mm, a thickness of 3.3mm, and a length of 3930 mm was degreased by a commercially availablealkali degreasing agent and pickled to remove the rust, then the steelpipe was sequentially dipped in a treatment solution obtained bydispersing titanium colloid in water (Prepalene Z made by NihonParkerizing Co. Ltd.) and a calcium zinc phosphate treatment solution(Palbond P made by Nihon Parkerizing Co. Ltd.) and dried by hot airheating to form a chemical treatment coating. The amount of depositionof the chemical treatment coating was 4 g/m². Next, an epoxy resinpowder primer (Powdax E made by Nippon Paint Co. Ltd.) was coated on theinner surface of the steel pipe at room temperature by electrostaticspraying, then the result was heated to 180° C. in a hot air heatingfurnace to form an epoxy primer layer. The thickness of the epoxy primerlayer was 100 μm. Further, using a two-layer round die, when shaping apolyethylene plastic pipe having an outside diameter of 42.4 mm, athickness of 1.5 mm, and a length of 4040 mm (melt start temperature of120° C.), an adhesive made of the ethylene/maleic anhydride copolymer(melt end temperature: 100° C.) was coated on the outer surface bycoextrusion to form an adhesive layer. The thickness of the adhesivelayer was 200 μm.

Thereafter, the polyethylene plastic pipe was inserted into the steelpipe and the steel pipe was roll drawn so that the outside diameter ofthe polyethylene plastic pipe was reduced by 1.4%, whereby thepolyethylene plastic pipe was made to closely contact the inner surfaceof the steel pipe, then the result was heated to 115° C. in a hot airheating furnace. The part of the polyethylene plastic pipe protrudingfrom the end portion of the steel pipe was cut off. The outer surface ofthis inner surface plastic lined steel pipe was degreased by acommercially available alkali degreasing agent, grit blasted to removethe rust, then coated with a commercially available organic zinc richpaint to a thickness of 75 μm and further coated with a commerciallyavailable clear paint to a thickness of 30 μm.

EXAMPLE 8

A steel pipe having an outside diameter of 50.8 mm, a thickness of 3.3mm, and a length of 3930 mm was degreased by a commercially availablealkali degreasing agent and pickled to remove the rust, then the steelpipe was sequentially dipped in a treatment solution obtained bydispersing titanium colloid in water (Prepalene Z made by NihonParkerizing Co. Ltd.) and a calcium zinc phosphate treatment solution(Palbond P made by Nihon Parkerizing Co. Ltd.) and dried by hot airheating to form a chemical treatment coating. The amount of depositionof the chemical treatment coating was 4 g/m². Next, an epoxy resinpowder primer (Powdax E made by Nippon Paint Co. Ltd.) was coated on theinner surface of the steel pipe at room temperature by electrostaticspraying, then the result was heated to 180° C. in a hot air heatingfurnace to form an epoxy primer layer. The thickness of the epoxy primerlayer was 100 μm. Further, using a two-layer round die, when shaping apolyethylene plastic pipe having an outside diameter of 42.4 mm, athickness of 1.5 mm, and a length of 4040 mm (melt start temperature of120° C.), an adhesive made of an ethylene/maleic anhydride/acrylatecopolymer (melt end temperature: 100° C.) was coated on the outersurface by coextrusion to form an adhesive layer. The thickness of theadhesive layer was 200 μm.

Thereafter, the polyethylene plastic pipe was inserted into the steelpipe and the steel pipe was roll drawn so that the outside diameter ofthe polyethylene plastic pipe was reduced by 1.4%, whereby thepolyethylene plastic pipe was made to closely contact the inner surfaceof the steel pipe, then the result was heated to 115° C. in a hot airheating furnace. The part of the polyethylene plastic pipe protrudingfrom the end portion of the steel pipe was cut off. The outer surface ofthis inner surface plastic lined steel pipe was degreased by acommercially available alkali degreasing agent, grit blasted to removethe rust, then coated with a commercially available organic zinc richpaint to a thickness of 75 μm and further coated with a commerciallyavailable clear paint to a thickness of 30 μm.

EXAMPLE 9

A steel pipe having an outside diameter of 50.8 mm, a thickness of 3.3mm, and a length of 3930 mm was degreased by a commercially availablealkali degreasing agent and pickled to remove the rust, then the steelpipe was sequentially dipped in a treatment solution obtained bydispersing titanium colloid in water (Prepalene Z made by NihonParkerizing Co. Ltd.) and a calcium zinc phosphate treatment solution(Palbond P made by Nihon Parkerizing Co. Ltd.) and dried by hot airheating to form a chemical treatment coating. The amount of depositionof the chemical treatment coating was 4 g/m². Next, an epoxy resinpowder primer (Powdax E made by Nippon Paint Co. Ltd.) was coated on theinner surface of the steel pipe at room temperature by electrostaticspraying, then the result was heated to 180° C. in a hot air heatingfurnace to form an epoxy primer layer. The thickness of the epoxy primerlayer was 100 μm. Further, using a two-layer round die, when shaping apolyethylene plastic pipe having an outside diameter of 42.4 mm, athickness of 1.5 mm, and a length of 4040 mm (melt start temperature of120° C.), an adhesive made of the ethylene/maleic anhydride/acrylateester copolymer (melt end temperature: 100° C.) was coated on the outersurface by coextrusion to form an adhesive layer. The thickness of theadhesive layer was 200 μm.

Thereafter, the polyethylene plastic pipe was inserted into the steelpipe and the steel pipe was roll drawn so that the outside diameter ofthe polyethylene plastic pipe was reduced by 1.4%, whereby thepolyethylene plastic pipe was made to closely contact the inner surfaceof the steel pipe, then the result was heated to 115° C. in a hot airheating furnace. The part of the polyethylene plastic pipe protrudingfrom the end portion of the steel pipe was cut off. The outer surface ofthis inner surface plastic lined steel pipe was degreased by acommercially available alkali degreasing agent, grit blasted to removethe rust, then coated with a commercially available organic zinc richpaint to a thickness of 75 μm and further coated with a commerciallyavailable clear paint to a thickness of 30 μm.

EXAMPLE 10

A steel pipe having an outside diameter of 50.8 mm, a thickness of 3.3mm, and a length of 3930 mm was degreased by a commercially availablealkali degreasing agent and pickled to remove the rust, then the steelpipe was sequentially dipped in a treatment solution obtained bydispersing titanium colloid in water (Prepalene Z made by NihonParkerizing Co. Ltd.) and a calcium zinc phosphate treatment solution(Palbond P made by Nihon Parkerizing Co. Ltd.) and dried by hot airheating to form a chemical treatment coating. The amount of depositionof the chemical treatment coating was 4 g/m². Next, an epoxy resinpowder primer (Powdax E made by Nippon Paint Co. Ltd.) was coated on theinner surface of the steel pipe at room temperature by electrostaticspraying, then the result was heated to 180° C. in a hot air heatingfurnace to form an epoxy primer layer. The thickness of the epoxy primerlayer was 100 μm. Further, using a two-layer round die, when shaping apolyethylene plastic pipe having an outside diameter of 42.4 mm, athickness of 1.5 mm, and a length of 4040 mm (melt start temperature of120° C.), an adhesive made of the ethylene/acrylate copolymer (melt endtemperature: 100° C.) was coated on the outer surface by coextrusion toform an adhesive layer. The thickness of the adhesive layer was 200 μm.

Thereafter, the polyethylene plastic pipe was inserted into the steelpipe and the steel pipe was roll drawn so that the outside diameter ofthe polyethylene plastic pipe was reduced by 1.4%, whereby thepolyethylene plastic pipe was made to closely contact the inner surfaceof the steel pipe, then the result was heated to 115° C. in a hot airheating furnace. The part of the polyethylene plastic pipe protrudingfrom the end portion of the steel pipe was cut off. The outer surface ofthis inner surface plastic lined steel pipe was degreased by acommercially available alkali degreasing agent, grit blasted to removethe rust, then coated with a commercially available organic zinc richpaint to a thickness of 75 μm and further coated with a commerciallyavailable clear paint to a thickness of 30 μm.

EXAMPLE 11

A steel pipe having an outside diameter of 50.8 mm, a thickness of 3.3mm, and a length of 3930 mm was degreased by a commercially availablealkali degreasing agent and pickled to remove the rust, then the steelpipe was sequentially dipped in a treatment solution obtained bydispersing titanium colloid in water (Prepalene Z made by NihonParkerizing Co. Ltd.) and a calcium zinc phosphate treatment solution(Palbond P made by Nihon Parkerizing Co. Ltd.) and dried by hot airheating to form a chemical treatment coating. The amount of depositionof the chemical treatment coating was 4 g/m². Next, an epoxy resinpowder primer (Powdax E made by Nippon Paint Co. Ltd.) was coated on theinner surface of the steel pipe at room temperature by electrostaticspraying, then the result was heated to 180° C. in a hot air heatingfurnace to form an epoxy primer layer. The thickness of the epoxy primerlayer was 100 μm. Further, using a two-layer round die, when shaping apolyethylene plastic pipe having an outside diameter of 42.4 mm, athickness of 1.5 mm, and a length of 4040 mm (melt start temperature of120° C.), an adhesive made of the ethylene/acrylate ester copolymer(melt end temperature: 100° C.) was coated on the outer surface bycoextrusion to form an adhesive layer. The thickness of the adhesivelayer was 200 μm.

Thereafter, the polyethylene plastic pipe was inserted into the steelpipe and the steel pipe was roll drawn so that the outside diameter ofthe polyethylene plastic pipe was reduced by 1.4%, whereby thepolyethylene plastic pipe was made to closely contact the inner surfaceof the steel pipe, then the result was heated to 115° C. in a hot airheating furnace. The part of the polyethylene plastic pipe protrudingfrom the end portion of the steel pipe was cut off. The outer surface ofthis inner surface plastic lined steel pipe was degreased by acommercially available alkali degreasing agent, grit blasted to removethe rust, then coated with a commercially available organic zinc richpaint to a thickness of 75 μm and further coated with a commerciallyavailable clear paint to a thickness of 30 μm.

EXAMPLE 12

A steel pipe having an outside diameter of 50.8 mm, a thickness of 3.3mm, and a length of 3930 mm was degreased by a commercially availablealkali degreasing agent and pickled to remove the rust, then the steelpipe was sequentially dipped in a treatment solution obtained bydispersing titanium colloid in water (Prepalene Z made by NihonParkerizing Co. Ltd.) and a calcium zinc phosphate treatment solution(Palbond P made by Nihon Parkerizing Co. Ltd.) and dried by hot airheating to form a chemical treatment coating. The amount of depositionof the chemical treatment coating was 4 g/m². Next, an epoxy resinpowder primer (Powdax E made by Nippon Paint Co. Ltd.) was coated on theinner surface of the steel pipe at room temperature by electrostaticspraying, then the result was heated to 180° C. in a hot air heatingfurnace to form an epoxy primer layer. The thickness of the epoxy primerlayer was 100 μm. Further, using a two-layer round die, when shaping apolyethylene plastic pipe having an outside diameter of 42.4 mm, athickness of 1.5 mm, and a length of 4040 mm (melt start temperature of120° C.), an adhesive made of an ethylene/methacrylate copolymer (meltend temperature: 100° C.) was coated on the outer surface by coextrusionto form an adhesive layer. The thickness of the adhesive layer was 200μm.

Thereafter, the polyethylene plastic pipe was inserted into the steelpipe and the steel pipe was roll drawn so that the outside diameter ofthe polyethylene plastic pipe was reduced by 1.4%, whereby thepolyethylene plastic pipe was made to closely contact the inner surfaceof the steel pipe, then the result was heated to 115° C. in a hot airheating furnace. The part of the polyethylene plastic pipe protrudingfrom the end portion of the steel pipe was cut off. The outer surface ofthis inner surface plastic lined steel pipe was degreased by acommercially available alkali degreasing agent, grit blasted to removethe rust, then coated with a commercially available organic zinc richpaint to a thickness of 75 μm and further coated with a commerciallyavailable clear paint to a thickness of 30 μm.

EXAMPLE 13

A steel pipe having an outside diameter of 50.8 mm, a thickness of 3.3mm, and a length of 3930 mm was degreased by a commercially availablealkali degreasing agent and pickled to remove the rust, then the steelpipe was sequentially dipped in a treatment solution obtained bydispersing titanium colloid in water (Prepalene Z made by NihonParkerizing Co. Ltd.) and a calcium zinc phosphate treatment solution(Palbond P made by Nihon Parkerizing Co. Ltd.) and dried by hot airheating to form a chemical treatment coating. The amount of depositionof the chemical treatment coating was 4 g/m². Next, an epoxy resinpowder primer (Powdax E made by Nippon Paint Co. Ltd.) was coated on theinner surface of the steel pipe at room temperature by electrostaticspraying, then the result was heated to 180° C. in a hot air heatingfurnace to form an epoxy primer layer. The thickness of the epoxy primerlayer was 100 μm. Further, using a two-layer round die, when shaping apolyethylene plastic pipe having an outside diameter of 42.4 mm, athickness of 1.5 mm, and a length of 4040 mm (melt start temperature of120° C.), an adhesive made of the ethylene/vinyl acetate copolymer (meltend temperature: 100° C.) was coated on the outer surface by coextrusionto form an adhesive layer. The thickness of the adhesive layer was 200μm.

Thereafter, the polyethylene plastic pipe was inserted into the steelpipe and the steel pipe was roll drawn so that the outside diameter ofthe polyethylene plastic pipe was reduced by 1.4%, whereby thepolyethylene plastic pipe was made to closely contact the inner surfaceof the steel pipe, then the result was heated to 115° C. in a hot airheating furnace. The part of the polyethylene plastic pipe protrudingfrom the end portion of the steel pipe was cut off. The outer surface ofthis inner surface plastic lined steel pipe was degreased by acommercially available alkali degreasing agent, grit blasted to removethe rust, then coated with a commercially available organic zinc richpaint to a thickness of 75 μm and further coated with a commerciallyavailable clear paint to a thickness of 30 μm.

EXAMPLE 14

A steel pipe having an outside diameter of 50.8 mm, a thickness of 3.3mm, and a length of 3930 mm was degreased by a commercially availablealkali degreasing agent and pickled to remove the rust, then the steelpipe was sequentially dipped in a treatment solution obtained bydispersing titanium colloid in water (Prepalene Z made by NihonParkerizing Co. Ltd.) and a calcium zinc phosphate treatment solution(Palbond P made by Nihon Parkerizing Co. Ltd.) and dried by hot airheating to form a chemical treatment coating. The amount of depositionof the chemical treatment coating was 4 g/m². Next, an epoxy resinpowder primer (Powdax E made by Nippon Paint Co. Ltd.) was coated on theinner surface of the steel pipe at room temperature by electrostaticspraying, then the result was heated to 180° C. in a hot air heatingfurnace to form an epoxy primer layer. The thickness of the epoxy primerlayer was 100 μm. Further, using a two-layer round die, when shaping apolyethylene plastic pipe having an outside diameter of 42.4 mm, athickness of 1.5 mm, and a length of 4040 mm (melt start temperature of120° C.), an adhesive made of an ionomer (melt end temperature: 100° C.)was coated on the outer surface by coextrusion to form an adhesivelayer. The thickness of the adhesive layer was 200 μm.

Thereafter, the polyethylene plastic pipe was inserted into the steelpipe and the steel pipe was roll drawn so that the outside diameter ofthe polyethylene plastic pipe was reduced by 1.4%, whereby thepolyethylene plastic pipe was made to closely contact the inner surfaceof the steel pipe, then the result was heated to 115° C. in a hot airheating furnace. The part of the polyethylene plastic pipe protrudingfrom the end portion of the steel pipe was cut off. The outer surface ofthis inner surface plastic lined steel pipe was degreased by acommercially available alkali degreasing agent, grit blasted to removethe rust, then coated with a commercially available organic zinc richpaint to a thickness of 75 μm and further coated with a commerciallyavailable clear paint to a thickness of 30 μm.

EXAMPLE 15

A steel pipe having an outside diameter of 50.8 mm, a thickness of 3.3mm, and a length of 3930 mm was degreased by a commercially availablealkali degreasing agent and pickled to remove the rust, then the steelpipe was sequentially dipped in a treatment solution obtained bydispersing titanium colloid in water (Prepalene Z made by NihonParkerizing Co. Ltd.) and a calcium zinc phosphate treatment solution(Palbond P made by Nihon Parkerizing Co. Ltd.) and dried by hot airheating to form a chemical treatment coating. The amount of depositionof the chemical treatment coating was 4 g/m². Next, using a two-layerround die, when shaping a cross-linked polyethylene plastic pipe havingan outside diameter of 42.4 mm, a thickness of 1.5 mm, and a length of4040 mm (melt start temperature of 120° C.), an adhesive made of themaleic anhydride-modified copolymer (melt end temperature: 100° C.) wascoated on the outer surface by coextrusion to form an adhesive layer.The thickness of the adhesive layer was 200 μm.

Thereafter, the cross-linked polyethylene plastic pipe was inserted intothe steel pipe and the steel pipe was roll drawn so that the outsidediameter of the cross-linked polyethylene plastic pipe was reduced by1.4%, whereby the cross-linked polyethylene plastic pipe was made toclosely contact the inner surface of the steel pipe, then the result washeated to 115° C. in a hot air heating furnace. The cross-linkedpolyethylene plastic pipe protruding from the end portion of the steelpipe was cut off. The outer surface of this inner surface plastic linedsteel pipe was degreased by a commercially available alkali degreasingagent, grit blasted to remove the rust, then coated with a commerciallyavailable alkyd-based paint to a thickness of 25 μm.

EXAMPLE 16

A steel pipe having an outside diameter of 50.8 mm, a thickness of 3.3mm, and a length of 3930 mm was degreased by a commercially availablealkali degreasing agent and pickled to remove the rust, then the steelpipe was sequentially dipped in a treatment solution obtained bydispersing titanium colloid in water (Prepalene Z made by NihonParkerizing Co. Ltd.) and a calcium zinc phosphate treatment solution(Palbond P made by Nihon Parkerizing Co. Ltd.) and dried by hot airheating to form-a chemical treatment coating. The amount of depositionof the chemical treatment coating was 4 g/m². Next, an epoxy resinpowder primer (Powdax E made by Nippon Paint Co. Ltd.) was coated on theinner surface of the steel pipe at room temperature by electrostaticspraying, then the result was heated to 180° C. in a hot air heatingfurnace to form an epoxy primer layer. The thickness of the epoxy primerlayer was 100 μm. Further, using a two-layer round die, when shaping across-linked polyethylene plastic pipe having an outside diameter of42.4 mm, a thickness of 1.5 mm, and a length of 4040 mm (melt starttemperature of 120° C.), an adhesive made of a maleic anhydride-modifiedpolyethylene (melt end temperature: 100° C.) was coated on the outersurface by coextrusion to form an adhesive layer. The thickness of theadhesive layer was 200 μm.

Thereafter, the cross-linked polyethylene plastic pipe was inserted intothe steel pipe and the steel pipe was roll drawn so that the outsidediameter of the cross-linked polyethylene plastic pipe was reduced by1.4%, whereby the cross-linked polyethylene plastic pipe was made toclosely contact the inner surface of the steel pipe, then the result washeated to 115° C. in a hot air heating furnace. The cross-linkedpolyethylene plastic pipe protruding from the end portion of the steelpipe was cut off. The outer surface of this inner surface plastic linedsteel pipe was degreased by a commercially available alkali degreasingagent, grit blasted to remove the rust, then coated with a commerciallyavailable alkyd-based paint to a thickness of 25 μm.

EXAMPLE 17

A steel pipe having an outside diameter of 50.8 mm, a thickness of 3.3mm, and a length of 3930 mm was degreased by a commercially availablealkali degreasing agent and pickled to remove the rust, then the steelpipe was sequentially dipped in a treatment solution obtained bydispersing titanium colloid in water (Prepalene Z made by NihonParkerizing Co. Ltd.) and a calcium zinc phosphate treatment solution(Palbond P made by Nihon Parkerizing Co. Ltd.) and dried by hot airheating to form a chemical treatment coating. The amount of depositionof the chemical treatment coating was 4 g/m². Next, an epoxy resinpowder primer (Powdax E made by Nippon Paint Co. Ltd.) was coated on theinner surface of the steel pipe at room temperature by electrostaticspraying, then the result was heated to 180° C. in a hot air heatingfurnace to form an epoxy primer layer. The thickness of the epoxy primerlayer was 100 μm. Further, using a two-layer round die, when shaping across-linked polyethylene plastic pipe having an outside diameter of42.4 mm, a thickness of 1.5 mm, and a length of 4040 mm (melt starttemperature of 120° C.), an adhesive made of an itaconicanhydride-modified polyethylene (melt end temperature: 100° C.) wascoated on the outer surface by coextrusion to form an adhesive layer.The thickness of the adhesive layer was 200 μm.

Thereafter, the cross-linked polyethylene plastic pipe was inserted intothe steel pipe and the steel pipe was roll drawn so that the outsidediameter of the cross-linked polyethylene plastic pipe was reduced by1.4%, whereby the cross-linked polyethylene plastic pipe was made toclosely contact the inner surface of the steel pipe, then the result washeated to 115° C. in a hot air heating furnace. The cross-linkedpolyethylene plastic pipe protruding from the end portion of the steelpipe was cut off. The outer surface of this inner surface plastic linedsteel pipe was degreased by a commercially available alkali degreasingagent, grit-blasted to remove the rust, then coated with a commerciallyavailable alkyd-based paint to a thickness of 25 μm.

EXAMPLE 18

A steel pipe having an outside diameter of 50.8 mm, a thickness of 3.3mm, and a length of 3930 mm was degreased by a commercially availablealkali degreasing agent and pickled to remove the rust, then the steelpipe was sequentially dipped in a treatment solution obtained bydispersing titanium colloid in water (Prepalene Z made by NihonParkerizing Co. Ltd.) and a calcium zinc phosphate treatment solution(Palbond P made by Nihon Parkerizing Co. Ltd.) and dried by hot airheating to form a chemical treatment coating. The amount of depositionof the chemical treatment coating was 4 g/m². Next, an epoxy resinpowder primer (Powdax E made by Nippon Paint Co. Ltd.) was coated on theinner surface of the steel pipe at room temperature by electrostaticspraying, then the result was heated to 180° C. in a hot air heatingfurnace to form an epoxy primer layer. The thickness of the epoxy primerlayer was 100 μm. Further, using a two-layer round die, when shaping across-linked polyethylene plastic pipe having an outside diameter of42.4 mm, a thickness of 1.5 mm, and a length of 4040 mm (melt starttemperature of 120° C.), an adhesive made of the ethylene/maleicanhydride copolymer (melt end temperature: 100° C.) was coated on theouter surface by coextrusion to form an adhesive layer. The thickness ofthe adhesive layer was 200 μm.

Thereafter, the cross-linked polyethylene plastic pipe was inserted intothe steel pipe and the steel pipe was roll drawn so that the outsidediameter of the cross-linked polyethylene plastic pipe was reduced by1.4%, whereby the cross-linked polyethylene plastic pipe was made toclosely contact the inner surface of the steel pipe, then the result washeated to 115° C. in a hot air heating furnace. The cross-linkedpolyethylene plastic pipe protruding from the end portion of the steelpipe was cut off. The outer surface of this inner surface plastic linedsteel pipe was degreased by a commercially available alkali degreasingagent, grit blasted to remove the rust, then coated with a commerciallyavailable alkyd-based paint to a thickness of 25 μm.

EXAMPLE 19

A steel pipe having an outside diameter of 50.8 mm, a thickness of 3.3mm, and a length of 3930 mm was degreased by a commercially availablealkali degreasing agent and pickled to remove the rust, then the steelpipe was sequentially dipped in a treatment solution obtained bydispersing titanium colloid in water (Prepalene Z made by NihonParkerizing Co. Ltd.) and a calcium zinc phosphate treatment solution(Palbond P made by Nihon Parkerizing Co. Ltd.) and dried by hot airheating to form a chemical treatment coating. The amount of depositionof the chemical treatment coating was 4 g/m². Next, an epoxy resinpowder primer (Powdax E made by Nippon Paint Co. Ltd.) was coated on theinner surface of the steel pipe at room temperature by electrostaticspraying, then the result was heated to 180° C. in a hot air heatingfurnace to form an epoxy primer layer. The thickness of the epoxy primerlayer was 100 μm. Further, using a two-layer round die, when shaping across-linked polyethylene plastic pipe having an outside diameter of42.4 mm, a thickness of 1.5 mm, and a length of 4040 mm (melt starttemperature of 120° C.), an adhesive made of an ethylene/maleicanhydride/acrylate copolymer (melt end temperature: 100° C.) was coatedon the outer surface by coextrusion to form an adhesive layer. Thethickness of the adhesive layer was 200 μm.

Thereafter, the cross-linked polyethylene plastic pipe was inserted intothe steel pipe and the steel pipe was roll drawn so that the outsidediameter of the cross-linked polyethylene plastic pipe was reduced by1.4%, whereby the cross-linked polyethylene plastic pipe was made toclosely contact the inner surface of the steel pipe, then the result washeated to 115° C. in a hot air heating furnace. The cross-linkedpolyethylene plastic pipe protruding from the end portion of the steelpipe was cut off. The outer surface of this inner surface plastic linedsteel pipe was degreased by a commercially available alkali degreasingagent, grit blasted to remove the rust, then coated with a commerciallyavailable alkyd-based paint to a thickness of 25 μm.

EXAMPLE 20

A steel pipe having an outside diameter of 50.8 mm, a thickness of 3.3mm, and a length of 3930 mm was degreased by a commercially availablealkali degreasing agent and pickled to remove the rust, then the steelpipe was sequentially dipped in a treatment solution obtained bydispersing titanium colloid in water (Prepalene Z made by NihonParkerizing Co. Ltd.) and a calcium zinc phosphate treatment solution(Palbond P made by Nihon Parkerizing Co. Ltd.) and dried by hot airheating to form a chemical treatment coating. The amount of depositionof the chemical treatment coating was 4 g/m². Next, an epoxy resinpowder primer (Powdax E made by Nippon Paint Co. Ltd.) was coated on theinner surface of the steel pipe at room temperature by electrostaticspraying, then the result was heated to 180° C. in a hot air heatingfurnace to form an epoxy primer layer. The thickness of the epoxy primerlayer was 100 μm. Further, using a two-layer round die, when shaping across-linked polyethylene plastic pipe having an outside diameter of42.4 mm, a thickness of 1.5 mm, and a length of 4040 mm (melt starttemperature of 120° C.), an adhesive made of an ethylene/maleicanhydride/acrylate ester copolymer (melt end temperature: 100° C.) wascoated on the outer surface by coextrusion to form an adhesive layer.The thickness of the adhesive layer was 200 μm.

Thereafter, the cross-linked polyethylene plastic pipe was inserted intothe steel pipe and the steel pipe was roll drawn so that the outsidediameter of the cross-linked polyethylene plastic pipe was reduced by1.4%, whereby the cross-linked polyethylene plastic pipe was made toclosely contact the inner surface of the steel pipe, then the result washeated to 115° C. in a hot air heating furnace. The cross-linkedpolyethylene plastic pipe protruding from the end portion of the steelpipe was cut off. The outer surface of this inner surface plastic linedsteel pipe was degreased by a commercially available alkali degreasingagent, grit blasted to remove the rust, then coated with a commerciallyavailable alkyd-based paint to a thickness of 25 μm.

EXAMPLE 21

A steel pipe having an outside diameter of 50.8 mm, a thickness of 3.3mm, and a length of 3930 mm was degreased by a commercially availablealkali degreasing agent and pickled to remove the rust, then the steelpipe was sequentially dipped in a treatment solution obtained bydispersing titanium colloid in water (Prepalene Z made by NihonParkerizing Co. Ltd.) and a calcium zinc phosphate treatment solution(Palbond P made by Nihon Parkerizing Co. Ltd.) and dried by hot airheating to form a chemical treatment coating. The amount of depositionof the chemical treatment coating was 4 g/m². Next, an epoxy resinpowder primer (Powdax E made by Nippon Paint Co. Ltd.) was coated on theinner surface of the steel pipe at room temperature by electrostaticspraying, then the result was heated to 180° C. in a hot air heatingfurnace to form an epoxy primer layer. The thickness of the epoxy primerlayer was 100 μm. Further, using a two-layer round die, when shaping across-linked polyethylene plastic pipe having an outside diameter of42.4 mm, a thickness of 1.5 mm, and a length of 4040 mm (melt starttemperature of 120° C.), an adhesive made of the ethylene/acrylatecopolymer (melt end temperature: 100° C.) was coated on the outersurface by coextrusion to form an adhesive layer. The thickness of theadhesive layer was 200 μm.

Thereafter, the cross-linked polyethylene plastic pipe was inserted intothe steel pipe and the steel pipe was roll drawn so that the outsidediameter of the cross-linked polyethylene plastic pipe was reduced by1.4%, whereby the cross-linked polyethylene plastic pipe was made toclosely contact the inner surface of the steel pipe, then the result washeated to 115° C. in a hot air heating furnace. The cross-linkedpolyethylene plastic pipe protruding from the end portion of the steelpipe was cut off. The outer surface of this inner surface plastic linedsteel pipe was degreased by a commercially available alkali degreasingagent, grit blasted to remove the rust, then coated with a commerciallyavailable alkyd-based paint to a thickness of 25 μm.

EXAMPLE 22

A steel pipe having an outside diameter of 50.8 mm, a thickness of 3.3mm, and a length of 3930 mm was degreased by a commercially availablealkali degreasing agent and pickled to remove the rust, then the steelpipe was sequentially dipped in a treatment solution obtained bydispersing titanium colloid in water (Prepalene Z made by NihonParkerizing Co. Ltd.) and a calcium zinc phosphate treatment solution(Palbond P made by Nihon Parkerizing Co. Ltd.) and dried by hot airheating to form a chemical treatment coating. The amount of depositionof the chemical treatment coating was 4 g/m². Next, an epoxy resinpowder primer (Powdax E made by Nippon Paint Co. Ltd.) was coated on theinner surface of the steel pipe at room temperature by electrostaticspraying, then the result was heated to 180° C. in a hot air heatingfurnace to form an epoxy primer layer. The thickness of the epoxy primerlayer was 100 μm. Further, using a two-layer round die, when shaping across-linked polyethylene plastic pipe having an outside diameter of42.4 mm, a thickness of 1.5 mm, and a length of 4040 mm (melt starttemperature of 120° C.), an adhesive made of an ethylene/acrylate estercopolymer (melt end temperature: 100° C.) was coated on the outersurface by coextrusion to form an adhesive layer. The thickness of theadhesive layer was 200 μm.

Thereafter, the cross-linked polyethylene plastic pipe was inserted intothe steel pipe and the steel pipe was roll drawn so that the outsidediameter of the cross-linked polyethylene plastic pipe was reduced by1.4%, whereby the cross-linked polyethylene plastic pipe was made toclosely contact the inner surface of the steel pipe, then the result washeated to 115° C. in a hot air heating furnace. The cross-linkedpolyethylene plastic pipe protruding from the end portion of the steelpipe was cut off. The outer surface of this inner surface plastic linedsteel pipe was degreased by a commercially available alkali degreasingagent, grit blasted to remove the rust, then coated with a commerciallyavailable alkyd-based paint to a thickness of 25 μm.

EXAMPLE 23

A steel pipe having an outside diameter of 50.8 mm, a thickness of 3.3mm, and a length of 3930 mm was degreased by a commercially availablealkali degreasing agent and pickled to remove the rust, then the steelpipe was sequentially dipped in a treatment solution obtained bydispersing titanium colloid in water (Prepalene Z made by NihonParkerizing Co. Ltd.) and a calcium zinc phosphate treatment solution(Palbond P made by Nihon Parkerizing Co. Ltd.) and dried by hot airheating to form a chemical treatment coating. The amount of depositionof the chemical treatment coating was 4 g/m². Next, an epoxy resinpowder primer (Powdax E made by Nippon Paint Co. Ltd.) was coated on theinner surface of the steel pipe at room temperature by electrostaticspraying, then the result was heated to 180° C. in a hot air heatingfurnace to form an epoxy primer layer. The thickness of the epoxy primerlayer was 100 μm. Further, using a two-layer round die, when shaping across-linked polyethylene plastic pipe having an outside diameter of42.4 mm, a thickness of 1.5 mm, and a length of 4040 mm (melt starttemperature of 120° C.), an adhesive made of an ethylene/methacrylatecopolymer (melt end temperature: 100° C.) was coated on the outersurface by coextrusion to form an adhesive layer. The thickness of theadhesive layer was 200 μm.

Thereafter, the cross-linked polyethylene plastic pipe was inserted intothe steel pipe and the steel pipe was roll drawn so that the outsidediameter of the cross-linked polyethylene plastic pipe was reduced by1.4%, whereby the cross-linked polyethylene plastic pipe was made toclosely contact the inner surface of the steel pipe, then the result washeated to 115° C. in a hot air heating furnace. The cross-linkedpolyethylene plastic pipe protruding from the end portion of the steelpipe was cut off. The outer surface of this inner surface plastic linedsteel pipe was degreased by a commercially available alkali degreasingagent, grit blasted to remove the rust, then coated with a commerciallyavailable alkyd-based paint to a thickness of 25 μm.

EXAMPLE 24

A steel pipe having an outside diameter of 50.8 mm, a thickness of 3.3mm, and a length of 3930 mm was degreased by a commercially availablealkali degreasing agent and pickled to remove the rust, then the steelpipe was sequentially dipped in a treatment solution obtained bydispersing titanium colloid in water (Prepalene Z made by NihonParkerizing Co. Ltd.) and a calcium zinc phosphate treatment solution(Palbond P made by Nihon Parkerizing Co. Ltd.) and dried by hot airheating to form a chemical treatment coating. The amount of depositionof the chemical treatment coating was 4 g/m². Next, an epoxy resinpowder primer (Powdax E made by Nippon Paint Co. Ltd.) was coated on theinner surface of the steel pipe at room temperature by electrostaticspraying, then the result was heated to 180° C. in a hot air heatingfurnace to form an epoxy primer layer. The thickness of the epoxy primerlayer was 100 μm. Further, using a two-layer round die, when shaping across-linked polyethylene plastic pipe having an outside diameter of42.4 mm, a thickness of 1.5 mm, and a length of 4040 mm (melt starttemperature of 120° C.), an adhesive made of an ethylene/vinyl acetatecopolymer (melt end temperature: 100° C.) was coated on the outersurface by coextrusion to form an adhesive layer. The thickness of theadhesive layer was 200 μm.

Thereafter, the cross-linked polyethylene plastic pipe was inserted intothe steel pipe and the steel pipe was roll drawn so that the outsidediameter of the cross-linked polyethylene plastic pipe was reduced by1.4%, whereby the cross-linked polyethylene plastic pipe was made toclosely contact the inner surface of the steel pipe, then the result washeated to 115° C. in a hot air heating furnace. The cross-linkedpolyethylene plastic pipe protruding from the end portion of the steelpipe was cut off. The outer surface of this inner surface plastic linedsteel pipe was degreased by a commercially available alkali degreasingagent, grit blasted to remove the rust, then coated with a commerciallyavailable alkyd-based paint to a thickness of 25 μm.

EXAMPLE 25

A steel pipe having an outside diameter of 50.8 mm, a thickness of 3.3mm, and a length of 3930 mm was degreased by a commercially availablealkali degreasing agent and pickled to remove the rust, then the steelpipe was sequentially dipped in a treatment solution obtained bydispersing titanium colloid in water (Prepalene Z made by NihonParkerizing Co. Ltd.) and a calcium zinc phosphate treatment solution(Palbond P made by Nihon Parkerizing Co. Ltd.) and dried by hot airheating to form a chemical treatment coating. The amount of depositionof the chemical treatment coating was 4 g/m². Next, an epoxy resinpowder primer (Powdax E made by Nippon Paint Co. Ltd.) was coated on theinner surface of the steel pipe at room temperature by electrostaticspraying, then the result was heated to 180° C. in a hot air heatingfurnace to form an epoxy primer layer. The thickness of the epoxy primerlayer was 100 μm. Further, using a two-layer round die, when shaping across-linked polyethylene plastic pipe having an outside diameter of42.4 mm, a thickness of 1.5 mm, and a length of 4040 mm (melt starttemperature of 120° C.), an adhesive made of an ionomer (melt endtemperature: 100° C.) was coated on the outer surface by coextrusion toform an adhesive layer. The thickness of the adhesive layer was 200 μm.

Thereafter, the cross-linked polyethylene plastic pipe was inserted intothe steel pipe and the steel pipe was roll drawn so that the outsidediameter of the cross-linked polyethylene plastic pipe was reduced by1.4%, whereby the cross-linked polyethylene plastic pipe was made toclosely contact the inner surface of the steel pipe, then the result washeated to 115° C. in a hot air heating furnace. The cross-linkedpolyethylene plastic pipe protruding from the end portion of the steelpipe was cut off. The outer surface of this inner surface plastic linedsteel pipe was degreased by a commercially available alkali degreasingagent, grit blasted to remove the rust, then coated with a commerciallyavailable alkyd-based paint to a thickness of 25 μm.

COMPARATIVE EXAMPLE 1

A steel pipe having an outside diameter of 34.0 mm, a thickness of 3.2mm, and a length of 4000 mm was degreased by a commercially availablealkali degreasing agent and pickled to remove the rust, then the steelpipe was sequentially dipped in a treatment solution obtained bydispersing titanium colloid in water (Prepalene Z made by NihonParkerizing Co. Ltd.) and a calcium zinc phosphate treatment solution(Palbond P made by Nihon Parkerizing Co. Ltd.) and dried by hot airheating to form a chemical treatment coating. The amount of depositionof the chemical treatment coating was 4 g/m². Next, an epoxy resinpowder primer (Powdax E made by Nippon Paint Co. Ltd.) was coated on theinner surface of the steel pipe at room temperature by electrostaticspraying, then the result was heated to 180° C. in a hot air heatingfurnace to form an epoxy primer layer. The thickness of the epoxy primerlayer was 100 μm. Further, using a two-layer round die, when shaping apolyethylene plastic pipe (melt start temperature: 120° C.), an adhesivemade of a maleic anhydride-modified polyethylene (melt end temperature:130° C.) was coated on the outer surface by co-extrusion to form anadhesive layer, then the pipe was drawn so as to be reduced by 13% inthe diameter direction so as to prepare a polyethylene plastic pipehaving an outside diameter of 26.1 mm, a thickness of 1.5 mm, and alength of 4500 mm. The thickness of the adhesive layer was 200 μm.

Thereafter, the polyethylene plastic pipe was inserted into the steelpipe and the result was heated to a steel pipe surface temperature of200° C. by high frequency induction heating so as to restore thepolyethylene plastic pipe in shape. The part of the polyethylene plasticpipe protruding from the end portion of the steel pipe was cut off.

COMPARATIVE EXAMPLE 2

A steel pipe having an outside diameter of 34.0 mm, a thickness of 3.2mm, and a length of 4000 mm was degreased by a commercially availablealkali degreasing agent and pickled to remove the rust, then the steelpipe was dipped in the calcium zinc phosphate treatment solution(Palbond P made by Nihon Parkerizing Co. Ltd.) and dried by hot airheating to form a chemical treatment coating. The amount of depositionof the chemical treatment coating was 4 g/m². Next, an epoxy resinpowder primer (Powdax E made by Nippon Paint Co. Ltd.) was coated on theinner surface of the steel pipe at room temperature by electrostaticspraying, then the result was heated to 180° C. in a hot air heatingfurnace to form an epoxy primer layer. The thickness of the epoxy primerlayer was 100 μm. Further, using a two-layer round die, at the time ofthe shaping of the polyethylene plastic pipe (melt start -temperature:120° C.), an adhesive made of a maleic anhydride-modified polyethylene(melt end temperature: 130° C.) was coated on the outer surface byco-extrusion to form an adhesive layer, then the pipe was drawn so as tobe reduced by 13% in the diameter direction so as to prepare apolyethylene plastic pipe having an outside diameter of 26.1 mm, athickness of 1.5 mm, and a length of 4500 mm. The thickness of theadhesive layer was 200 μm.

Thereafter, the polyethylene plastic pipe was inserted into the steelpipe and the result was heated to a steel pipe surface temperature of200° C. by high frequency induction heating to restore the polyethyleneplastic pipe in shape. The part of the polyethylene plastic pipeprotruding from the end portion of the steel pipe was cut off.

COMPARATIVE EXAMPLE 3

A steel pipe having an outside diameter of 34.0 mm, a thickness of 3.2mm, and a length of 4000 mm was degreased by a commercially availablealkali degreasing agent and pickled to remove the rust, then the steelpipe was sequentially dipped in a treatment solution obtained bydispersing titanium colloid in water (Prepalene Z made by NihonParkerizing co. Ltd.) and a calcium zinc phosphate treatment solution(Palbond P made by Nihon Parkerizing Co. Ltd.) and dried by hot airheating to form a chemical treatment coating. The amount of depositionof the chemical treatment coating was 4 g/m². Next, an epoxy resinpowder primer (Powdax E made by Nippon Paint Co. Ltd.) was coated on theinner surface of the steel pipe at room temperature by electrostaticspraying, then the result was heated to 180° C. in a hot air heatingfurnace to form an epoxy primer layer. The thickness of the epoxy primerlayer was 100 μm. Further, using a two-layer round die, at the time ofshaping a cross-linked polyethylene plastic pipe (melt starttemperature: 120° C.), an adhesive made of a maleic anhydride-modifiedpolyethylene (melt end temperature: 130° C.) was coated on the outersurface by coextrusion to form an adhesive layer, then the pipe wasdrawn so as to be reduced by 30% in the diameter direction so as toprepare a cross-linked polyethylene plastic pipe having an outsidediameter of 26.1 mm, a thickness of 1.5 mm, and a length of 4500 mm. Thethickness of the adhesive layer was 200 μm.

Thereafter, the cross-linked polyethylene plastic pipe was inserted intothe steel pipe and the result was heated to a steel pipe surfacetemperature of 200° C. by high frequency induction heating so as torestore the cross-linked polyethylene plastic pipe in shape. The part ofthe cross-linked polyethylene plastic pipe protruding from the endportion of the steel pipe was cut off.

COMPARATIVE EXAMPLE 4

A steel pipe having an outside diameter of 34.0 mm, a thickness of 3.2mm, and a length of 4000 mm was degreased by a commercially availablealkali degreasing agent and pickled to remove the rust, then the steelpipe was dipped in a calcium zinc phosphate treatment solution (PalbondP made by Nihon Parkerizing Co. Ltd.) and dried by hot air heating toform a chemical treatment coating. The amount of deposition of thechemical treatment coating was 4 g/m². Next, an epoxy resin powderprimer (Powdax E made by Nippon Paint Co. Ltd.) was coated on the innersurface of the steel pipe at room temperature by electrostatic spraying,then the result was heated to 180° C. in a hot air heating furnace toform an epoxy primer layer. The thickness of the epoxy primer layer was100 μm. Further, using a two-layer round die, at the time of shaping across-linked polyethylene plastic pipe (melt start temperature: 120°C.), an adhesive made of a maleic anhydride-modified polyethylene (meltend temperature: 130° C.) was coated on the outer surface byco-extrusion to form an adhesive layer, then the pipe was drawn so as tobe reduced by 30% in the diameter direction so as to prepare across-linked polyethylene plastic pipe having an outside diameter of26.1 mm, a thickness of 1.5 mm, and a length of 4500 mm. The thicknessof the adhesive layer was 200 μm.

Thereafter, the cross-linked polyethylene plastic pipe was inserted intothe steel pipe and the result was heated to a steel pipe surfacetemperature of 200° C. by high frequency induction heating so as torestore the cross-linked polyethylene plastic pipe in shape. The part ofthe cross-linked polyethylene plastic pipe protruding from the endportion of the steel pipe was cut off.

COMPARATIVE EXAMPLE 5

A steel pipe having an outside diameter of 50.8 mm, a thickness of 3.3mm, and a length of 3930 mm was degreased by a commercially availablealkali degreasing agent and pickled to remove the rust, then the steelpipe was dipped in a calcium zinc phosphate treatment solution (PalbondP made by Nihon Parkerizing Co. Ltd.) and dried by hot air heating toform a chemical treatment coating. The amount of deposition of thechemical treatment coating was 4 g/m², and the mean grain size thereofwas about 15 μm. Next, an epoxy resin powder primer (Powdax E made byNippon Paint Co. Ltd.) was coated on the inner surface of the steel pipeat room temperature by electrostatic spraying, then the result washeated to 180° C. in a hot air heating furnace to form an epoxy primerlayer. The thickness of the epoxy primer layer was 100 μm. Further,using a two-layer round die, when shaping a polyethylene plastic pipehaving an outside diameter of 42.4 mm, a thickness of 1.5 mm, and alength of 4040 mm (melt start temperature of 120° C.), an adhesive madeof a maleic anhydride-modified polyethylene (melt end temperature: 100°C.) was coated on the outer surface by coextrusion to form an adhesivelayer. The thickness of the adhesive layer was 200 μm.

Thereafter, the polyethylene plastic pipe was inserted into the steelpipe and the steel pipe was roll drawn so that the outside diameter ofthe polyethylene plastic pipe was reduced by 1.4%, whereby thepolyethylene plastic pipe was made to closely contact the inner surfaceof the steel pipe, then the result was heated to 115° C. in a hot airheating furnace. The part of the polyethylene plastic pipe protrudingfrom the end portion of the steel pipe was cut off.

COMPARATIVE EXAMPLE 6

A steel pipe having an outside diameter of 50.8 mm, a thickness of 3.3mm, and a length of 3930 mm was degreased by a commercially availablealkali degreasing agent and pickled to remove the rust, then the steelpipe was dipped in a calcium zinc phosphate treatment solution (PalbondP made by Nihon Parkerizing Co. Ltd.) and dried by hot air heating toform a chemical treatment coating. The amount of deposition of thechemical treatment coating was 4 g/m². Next, an epoxy resin powderprimer (Powdax E made by Nippon Paint Co. Ltd.) was coated on the innersurface of the steel pipe at room temperature by electrostatic spraying,then the result was heated to 180° C. in a hot air heating furnace toform an epoxy primer layer. The thickness of the epoxy primer layer was100 μm. Further, using a two-layer round die, when shaping across-linked polyethylene plastic pipe having an outside diameter of42.4 mm, a thickness of 1.5 mm, and a length of 4040 mm (melt starttemperature of 120° C.), an adhesive made of a maleic anhydride-modifiedpolyethylene (melt end temperature: 100° C.) was coated on the outersurface by coextrusion to form an adhesive layer. The thickness of theadhesive layer was 200 μm.

Thereafter, the cross-linked polyethylene plastic pipe was inserted intothe steel pipe and the steel pipe was roll drawn so that the outsidediameter of the cross-linked polyethylene plastic pipe was reduced by1.4%, whereby the cross-linked polyethylene plastic pipe was made toclosely contact the inner surface of the steel pipe, then the result washeated to 115° C. in a hot air heating furnace. The part of thecross-linked polyethylene plastic pipe protruding from the end portionof the steel pipe was cut off.

The plastic lined steel pipes of Examples 1 to 15 and ComparativeExamples 1 to 6 were measured for the shearing adhesion strength betweenthe steel pipe and the plastic pipe of the inner surface. The shearingadhesion strength was measured by cutting each produced plastic linedsteel pipes to pieces having lengths of 20 mm, supporting only the steelpipe portions by using a jig, and pushing out only the plastic lininglayers of the inner surface under conditions of 10 mm/min. The shearingadhesion strength was found by the pushing force at this time. Threesamples were extracted from each plastic lined steel pipe, and the meanvalue was found. The unit of the shearing adhesion strength is MPa. Thetemperature during the measurement was uniformly set to 23° C. Also, theshearing adhesion strengths after warm water of 60° C. and hot water of95° C. were passed through the plastic lined steel pipes for one yearwere measured together. The conditions and measurement results of theexamples are shown in Tables 1a, 1b, 2a, 2b, 3a, 3b, 4a, and 4b.

The initial shearing adhesion strengths of Examples 1 to 25 were allmore than 2.0 MPa and high values of the preferable range of 4.0 MPa. Itis also learned that the shearing adhesion strengths after warm water of60° C. and hot water of 95° C. were passed through the plastic linedsteel pipes for one year were remarkably high in comparison withComparative Examples 1 to 4.

Further, freezing/thawing tests envisioning use at cold locations werecarried out on the plastic lined steel pipes of the examples and thecomparative examples. The freezing/thawing tests were carried out bycutting the produced plastic lined steel pipes to pieces having lengthsof 150 mm, standing them up in vessels filled with tap water to immersethem up to about ⅓, placing the vessels in an isothermal tank andfreezing them so that the temperature became −20° C., then taking theseout and placing in an isothermal tank and thawing them so that thetemperature became 60° C. This operation was repeated 1500 times. Thenumbers of times until peeling of the plastic lining layers of the innersurfaces were measured. The measurement results thereof are also shownin Tables 1b, 2b, 3b, and 4b.

In all of Examples 1 to 25, no peeling occurred at the plastic lininglayer of the inner surface, but in Comparative Examples 1 to 6, peelingoccurred at the plastic lining layer of the inner surface after a smallnumber of times of this operation. TABLE 1a Steel pipe inner surfaceSubstrate Steel pipe Ex- Plastic treatment outer amples layer Adhesivelayer etc. surface Ex- Poly- Maleic Calcium zinc Zinc rich ample 1ethylene anhydride-modified phosphate* paint coating resin polyethyleneEx- Poly- Maleic Calcium zinc Galvanization ample 2 ethyleneanhydride-modified phosphate* + resin polyethylene epoxy resin powderprimer Ex- Poly- Maleic Calcium zinc Primary anti- ample 3 ethyleneanhydride-modified phosphate* + rust coating resin polyethylene epoxyresin powder primer Ex- Poly- Maleic Calcium zinc Zinc rich ample 4ethylene anhydride-modified phosphate* + paint coating resinpolyethylene epoxy resin powder primer Ex- Poly- Maleic Calcium zincPolyethylene ample 5 ethylene anhydride-modified phosphate* + coatingresin polyethylene epoxy resin powder primer*With treatment for grain refinement

TABLE 1b Shearing adhesion strength (MPa) After After Freezing/ carrying60° C. carrying 95° C. thawing test warm water hot water for Times untilExamples Initial for 1 year 1 year peeling Example 1 4.0 3.6 3.2 Nopeeling after 1500X Example 2 4.0 3.8 3.6 No peeling after 1500X Example3 4.0 3.8 3.6 No peeling after 1500X Example 4 4.0 3.8 3.6 No peelingafter 1500X Example 5 4.0 3.8 3.6 No peeling after 1500X

TABLE 2a Steel pipe inner surface Substrate Steel pipe Ex- Plastictreatment outer amples layer Adhesive layer etc. surface Ex- Poly-Itaconic anhydride- Calcium zinc Zinc rich ample 6 ethylene modifiedphosphate* + paint resin polyethylene epoxy resin coating powder primerEx- Poly- Ethylene/maleic Calcium zinc Zinc rich ample 7 ethyleneanhydride phosphate* + paint resin copolymer epoxy resin coating powderprimer Ex- Poly- Ethylene/maleic Calcium zinc Zinc rich ample 8 ethyleneanhydride/acrylate phosphate* + paint resin copolymer epoxy resincoating powder primer Ex- Poly- Ethylene/maleic calcium zinc Zinc richample 9 ethylene anhydride/acrylate phosphate* + paint resin estercopolymer epoxy resin coating powder primer Ex- Poly- Ethylene/acrylateCalcium zinc Zinc rich ample ethylene copolymer phosphate* + paint 10resin epoxy resin coating powder primer Ex- Poly- Ethylene/acrylateCalcium zinc Zinc rich ample ethylene ester copolymer phosphate* + paint11 resin epoxy resin coating powder primer Ex- Poly- Ethylene/ Calciumzinc Zinc rich ample ethylene methacrylate phosphate* + paint 12 resincopolymer epoxy resin coating powder primer Ex- Poly- Ethylene/vinylCalcium zinc Zinc rich ample ethylene acetate copolymer phosphate* +paint 13 resin epoxy resin coating powder primer Ex- Poly- IonomerCalcium zinc Zinc rich ample ethylene phosphate* + paint 14 resin epoxyresin coating powder primer*With treatment for grain refinement

TABLE 2b Shearing adhesion strength (MPa) After After Freezing/ carrying60° C. carrying 95° C. thawing test warm water hot water for Times untilExamples Initial for 1 year 1 year peeling Example 6 4.0 3.6 3.2 Nopeeling after 1500X Example 7 4.0 3.8 3.6 No peeling after 1500X Example8 4.0 3.8 3.6 No peeling after 1500X Example 9 4.0 3.8 3.6 No peelingafter 1500X Example 10 4.0 3.8 3.6 No peeling after 1500X Example 11 4.03.8 3.6 No peeling after 1500X Example 12 4.0 3.8 3.6 No peeling after1500X Example 13 4.0 3.8 3.6 No peeling after 1500X Example 14 4.0 3.83.6 No peeling after 1500X

TABLE 3a Steel pipe inner surface Substrate Steel pipe Ex- Plastictreatment outer amples layer Adhesive layer etc. surface Ex- Cross-Maleic Calcium zinc Primary ample linked anhydride-modified phosphate* +anti-rust 15 poly- polyethylene epoxy resin coating ethylene powderprimer resin Ex- Cross- Maleic Calcium zinc Primary ample linkedanhydride-modified phosphate* + anti-rust 16 poly- polyethylene epoxyresin coating ethylene powder primer resin Ex- Cross- Itaconic Calciumzinc Primary ample linked anhydride-modified phosphate* + anti-rust 17poly- polyethylene epoxy resin coating ethylene powder primer resin Ex-Cross- Ethylene/maleic Calcium zinc Primary ample linkedanhydride-modified phosphate* + anti-rust 18 poly- epoxy resin coatingethylene powder primer resin Ex- Cross- Ethylene/maleic Calcium zincPrimary ample linked anhydride-modified phosphate* + anti-rust 19 poly-copolymer epoxy resin coating ethylene powder primer resin Ex- Cross-Ethylene/maleic Calcium zinc Primary ample linked anhydride-modifiedphosphate* + anti-rust 20 poly- ester copolymer epoxy resin coatingethylene powder primer resin Ex- Cross- Ethylene/acrylate Calcium zincPrimary ample linked copolymer phosphate* + anti-rust 21 poly- epoxyresin coating ethylene powder primer resin Ex- Cross- Ethylene/acrylateCalcium zinc Primary ample linked ester copolymer phosphate* + anti-rust22 poly- epoxy resin coating ethylene powder primer resin Ex- Cross-Ethylene/ Calcium zinc Primary ample linked methacrylate phosphate* +anti-rust 23 poly- copolymer epoxy resin coating ethylene powder primerresin Ex- Cross- Ethylene/vinyl Calcium zinc Primary ample linkedacetate copolymer phosphate* + anti-rust 24 poly- epoxy resin coatingethylene powder primer resin Ex- Cross- Ionomer Calcium zinc Primaryample linked phosphate* + anti-rust 25 poly- epoxy resin coatingethylene powder primer resin*With treatment for grain refinement

TABLE 3b Shearing adhesion strength (MPa) After After Freezing/ carrying60° C. carrying 95° C. thawing test warm water hot water for Times untilExamples Initial for 1 year 1 year peeling Example 4.0 3.6 3.2 Nopeeling 15 after 1500X Example 4.0 3.8 3.6 No peeling 16 after 1500XExample 4.0 3.8 3.6 No peeling 17 after 1500X Example 4.0 3.8 3.6 Nopeeling 18 after 1500X Example 4.0 3.8 3.6 No peeling 19 after 1500XExample 4.0 3.8 3.6 No peeling 20 after 1500X Example 4.0 3.8 3.6 Nopeeling 21 after 1500X Example 4.0 3.8 3.6 No peeling 22 after 1500XExample 4.0 3.8 3.6 No peeling 23 after 1500X Example 4.0 3.8 3.6 Nopeeling 24 after 1500X Example 4.0 3.8 3.6 No peeling 25 after 1500X

TABLE 4a Steel pipe inner surface Substrate Steel pipe Ex- Plastictreatment outer amples layer Adhesive layer etc. surface Comp. Poly-Maleic Calcium zinc — Ex. 1 ethylene anhydride-modified phosphate* +resin polyethylene epoxy resin powder primer Comp. Poly- Maleic Calciumzinc — Ex. 2 ethylene anhydride-modified phosphate** + resinpolyethylene epoxy resin powder primer Comp. Cross- Maleic Calcium zinc— Ex. 3 linked anhydride-modified phosphate* + poly- polyethylene epoxyresin ethylene powder primer resin Comp. Cross- Maleic Calcium zinc —Ex. 4 linked anhydride-modified phosphate** + poly- polyethylene epoxyresin ethylene powder primer resin Comp. Poly- Maleic Calcium zinc — Ex.5 ethylene anhydride-modified phosphate** + resin polyethylene epoxyresin powder primer Comp. Cross- Maleic Calcium zinc — Ex. 6 linkedanhydride-modified phosphate** + poly- polyethylene epoxy resin ethylenepowder primer resin*With treatment for grain refinement**No treatment for grain refinement

TABLE 4b Shearing adhesion strength (MPa) After After Freezing/ carrying60° C. carrying 95° C. thawing test warm water hot water for Times untilExamples Initial for 1 year 1 year peeling Comp. 3.2 1.6 0.8 No peelingEx. 1 after 300X Comp. 1.6 0.8 0.4 Peeling Ex. 2 after 150X Comp. 3.21.6 0.8 Peeling Ex. 3 after 300X Comp. 1.6 0.8 0.4 Peeling Ex. 4 after150X Comp. 4.0 3.8 3.6 Peeling Ex. 5 after 500X Comp. 4.0 3.8 3.6Peeling Ex. 6 after 500X

It was learned from these tables that the plastic lined steel pipes ofthe present invention are excellent in adhesiveness between the steelpipe and the inner surface plastic lining layer over a long period evenat cold locations.

INDUSTRIAL APPLICABILITY

According to the present invention, by drawing the steel pipe to linethe inside surface while leaving an expansion force whereby the outsidediameter of the plastic pipe tends to become larger than the insidediameter of the steel pipe and further providing between the steel pipeand the plastic pipe an adhesive layer and a phosphate chemicaltreatment coating treated for grain refinement to reinforce the adhesionand providing an epoxy primer layer according to need, a plastic linedsteel pipe excellent in the adhesion between the steel pipe and theinner surface plastic lining layer over a long period even at coldlocations and usable for piping for water supply, hot water supply,air-conditioning, firefighting, drainage, etc. can be provided.

1. A plastic lined steel pipe characterized by having an adhesive layeron an inner surface of a steel pipe or a steel pipe galvanized on itsouter surface, having a plastic layer on its further inner side, andhaving an initial shearing adhesion strength between the steel pipe andthe plastic layer of 2.0 MPa or more, said steel pipe being a steel pipegiven substrate treatment on its inner surface in advance, saidsubstrate treatment comprising forming a phosphate chemical treatmentcoating treated for grain refinement.
 2. A plastic lined steel pipe asset forth in claim 1, wherein said plastic layer is a polyolefin resinor a cross-linked polyolefin resin.
 3. A plastic lined steel pipe as setforth in claim 1, wherein said adhesive layer is comprised of one ormore of a maleic anhydride-modified polyolefin, itaconicanhydride-modified polyolefin, ethylene/maleic anhydride copolymer,ethylene/maleic anhydride/acrylate copolymer, ethylene/maleicanhydride/acrylate ester copolymer, ethylene/acrylate copolymer,ethylene/acrylate ester copolymer, ethylene/methacrylate copolymer,ethylene/vinyl acetate copolymer, and ionomer, and a melt endtemperature of the adhesive layer is over a usage temperature of saidplastic layer and less than a melt start temperature.
 4. A plastic linedsteel pipe as set forth in claim 1, wherein an epoxy primer layer isprovided between said steel pipe and said adhesive layer.
 5. A plasticlined steel pipe as set forth in claim 1, wherein a primary anti-rustcoating, a zinc rich paint coating, or a polyolefin coating is providedon the outer surface of said plastic lined steel pipe instead ofgalvanization.
 6. A method for producing a plastic lined steel pipe asset forth in claim 1, comprising, when producing said plastic linedsteel pipe, applying substrate treatment to a steel pipe or applyingsubstrate treatment to a steel pipe, then applying an epoxy primerlayer, inserting a plastic pipe having an outside diameter smaller thanthe inside diameter of the steel pipe and having an adhesive layer onits outer surface into said steel pipe, drawing the steel pipe so as tomake the plastic pipe come in close contact with the steel pipe, thenheating the result at a temperature not less than a melt end temperatureof the adhesive layer and less than a melt start temperature of theplastic pipe.
 7. A method for producing a plastic lined steel pipe asset forth in claim 6, further comprising, when drawing said steel pipe,drawing the steel pipe so that the outside diameter of the plastic pipeis reduced by 0.5 to 10%.